Recent developments in methods for generating and detecting terahertz radiation have produced an interest in using terahertz frequency spectrum data for detecting the presence of chemicals relatively unobtrusively. For example, certain chemicals can be identified by the frequency of their absorption spectrum resonance in the terahertz range using Terahertz Time-Domain Spectroscopy (THz-TDS). Typically, in THz-TDS a sequence of femtosecond pulses from a mode-locked laser are focused onto a semiconductor that is configured to produce THz radiation. Early methods and apparatus for terahertz imaging are described in U.S. Pat. No. 5,623,145, to Nuss, and in U.S. Pat. No. 5,710,145, to Nuss, both of which are hereby incorporated herein by reference in their entireties.
Terahertz (THz) radiation is directed to the desired target, and a reflection or transmission signal is detected and analyzed. The detected signal is a time-dependent signal, and is therefore transformed, e.g., with a Fourier transform, to obtain frequency-dependent spectral information. The THz spectral information can sometimes be used to identify particular chemical compositions. For example, certain explosives have unique spectral characteristics in the THz region that may be amenable to standoff detection.
In Detection and identification of explosives using terahertz pulsed spectroscopic imaging, Y. C. Shen et al., Appl. Phys. Lett. 86, 241116 (2005) THz-TDS, hereby incorporated by reference, the authors demonstrate using reflection terahertz measurements detection of the absorption spectrum of a particular explosive (RDX).
In particular, spectroscopic methods using terahertz radiation have several unique properties that provide advantages in certain applications. For example, terahertz radiation is non-ionizing. Also, many materials such as clothing, paper, and the like are substantially transparent at this frequency, while other materials, including plastics and ceramics, are readily visible in terahertz imaging. In particular, many chemicals of interest have a characteristic spectrum at terahertz frequencies that are amenable to detection by spectroscopic means.
However, surface roughness features of a target can cause electromagnetic scattering of terahertz waves that decreases the signal-to-noise of the spectral features. The noise can obscure the desired spectral signatures of chemicals at these frequencies. Electromagnetic waves are scattered when they encounter a rough surface having length scale features that are comparable to the wavelength of the wave. THz waves, having wavelengths on order of hundreds of microns, can also suffer from classical electromagnetic scattering caused by embedded internal void volumes or inhomogeneous granularity of the material. The bandwidth of THz pulses is reduced dramatically upon propagation through pellets of granular material. So far, little progress has been made in eliminating the negative effects of scattering. Y. C. Shen et al. showed that by averaging over 1800 disjoint transmission measurements, the granularity scattering effect can be effectively decreased (Elimination of scattering effects in spectral measurement of granulated materials using terahertz pulsed spectroscopy, Shen et al., Appl. Phys. Lett., Vol. 92, pp. 051103-3, 2008). Due to the strong absorption of terahertz radiation in transmission through thick layers of materials, and for other practical considerations, reflection geometries are more suitable for the stand-off discrimination of chemicals, as compared to transmission spectroscopy.
In the reflection mode, however, surface roughness is the dominant source of terahertz scattering. Y. Dikmelik et al. modeled the effects of surface roughness by considering the summation of a large number of elemental reflections with different time delays that correspond to the surface height variations at each element (Effect of surface roughness on reflection spectra obtained by terahertz time-domain spectroscopy, Y. Dikmelik et al., Opt. Lett., Vol. 31, pp. 2653-2655, 2006).
There remains a need for improved methods and systems for analyzing THz spectrum information to detect chemical compositions in target samples, and in particular for methods that overcome limitations due to surface scattering effects.